Objective: Our goal is the effective treatment of leukemia with potent, specific antisera, using a mouse model system. Other goals include work on diagnosis of early cancer of the prostate, and immunotherapy as an adjunct to conventional cancer therapy. Approach: We have recently developed methods for producing and analyzing antisera that show a relatively high specificity against leukemia cells (Reif, A.E. and Robinson, C.M., Immunology 28: 199-205, 1975). We hope to determine how these methods can be improved to the point where the specificity against leukemia cells is sufficiently high and the toxicity to the host is sufficiently low that such antisera can be used for effective therapy of experimental leukemias. As a necessary adjunct to this work, we are determining the conditions for in vitro absorptions under which the lysis of erythrocytes and of other cell types can be held to a minimum, for such lysates possess relatively high in vivo toxicity. Next, we hope to return to in vivo therapy experiments with tumor cells radiolabeled with IUDR-I125, in which we hope to attain results comparable with the rather good results we obtained with BCG, which paralleled the later clinical experience with BCG in melanoma.